The present invention relates to a process for determining the opening time of an injection valve of a common-rail injection system of the generic type in which the injection duration is determined from a characteristic diagram based on the fuel quantity to be injected and the static pressure in the common rail.
Common-rail injection systems of this type are generally known. European Patent Document EP 0 570 986 A2 discloses a system in which the injection operation is divided into two parts, so-called "pilot" and "main" injections. The opening time of the injection valve determined for conveying a defined quantity of fuel is distributed between the pilot injection and to the main injection. Control of the injection valve in the main injection period takes into account the fact that a remaining magnetic field from the pilot injection period still exists in the electromagnetic injection valve so that the controlling duration of the injection valve must be reduced correspondingly to convey the fuel quantity in the main injection period. The degree of such reduction is directly proportional to the duration of the pilot injection period and inversely proportional to the time interval of the pilot injection period and the main injection period. According to the statements in this published patent application, this approach also eliminates undesirable effects of compressional vibrations.
In the fuel injection process according to the invention, for the purpose of, reading values from a characteristic diagram, the value of the static pressure in the common rail is corrected by an amount which is determined from the vibration behavior of the fuel as a function of its compressibility, the quantity f fuel removed or the controlling duration of the injection valve during at least one preceding injection operation, and the time between at least one preceding injection operation and the actual injection operation.
The invention is advantageous with respect to the known state of the art in that the opening duration of the injection valve is better adapted to the variable pressure. That is, due to pressure fluctuations, the pressure will first fall and then rise, fluctuating beyond the value of the static pressure in the common-rail system. Thus, in order to be able to decide appropriately whether the opening time of the injection valve must be shortened or extended relative to the opening time which is appropriate in the case of static pressure in the common-rail system, the momentary time value of the pressure must be determined taking into account the compressional fluctuations. In this case, the time variation of the compressional vibrations shows in particular that, because of the reduction of the fuel quantity in the common rail system, there is first a lowering of the pressure. Thus, if the next injection operation follows so soon after the preceding injection that the pressure has not yet risen to exceed the static pressure value in the common rail, the opening time of the injection valve must be extended. In this manner, control of the injected fuel quantity relative to the demanded fuel quantity can be improved and the combustion operation optimized particularly with respect to fuel consumption and pollutant emissions.
In another embodiment of the invention the time variation of a compressional vibration is determined by at least a value which represents the compressibility of the fuel. In an idealized model, the compressional vibration can be represented as a vibration with a single frequency. However, in practice, it has been found that the vibration represents the superimposition of several vibrations of different frequencies.
The time variation of the vibration is therefore stored in a characteristic diagram, as a function of the fuel compressibility.
Thus, differences in the course of the pressure can advantageously be taken into account as a function of certain parameters, such as the fuel temperature. The compressibility of the fuel decisively determines the time variation of the compressional vibration.
In another embodiment of the invention, the at least one value which represents the compressibility is the speed of sound of the fuel, the modulus of elasticity of the fuel and/or the density of the fuel. Advantageously, it has been found that the compressibility of the fuel can easily be determined by means of comparatively easily accessible parameters.
In still another embodiment of the invention, the at least one value is determined by means of the static rail pressure and/or of the fuel temperature. As the result, the most important influences of the compressibility of the fuel can easily be taken into account.
In yet another embodiment, the phase position or a standardized value of the pressure deviation at the start of an injection operation is determined from the time period between at least one preceding injection operation and current injection which is to be carried out. In this case, it has been found that, with a separation of the two parameters describing the vibrations (amplitude--that is, maximal deflection--on the one hand, and the frequency or superimposition of several frequencies which determine the time variation of the compressional vibration on the other hand), the vibration can be stored in characteristic diagrams at acceptable expenditures. As a result, it is possible to represent numerically the compressional vibration, particularly with respect to the relevant values in real time. That is, the amplitude of the compressional vibrations on the one hand, and the vibration duration or a corresponding value from the superimposition of several vibration durations, are determined.
In still another embodiment of the invention, the amplitude of the compressional vibration, or a factor by which the standardized value of the pressure deviation is multiplied, is derived from the injected fuel quantity during at least one preceding injection operation. It has been found that, as a result, the amplitude or the factor can easily be determined by way of a corresponding characteristic curve.
In a further embodiment, the fuel quantity injected during at least one preceding injection operation is determined from the controlling duration of the valve during the at least one preceding injection operation. In this manner, the fuel quantity can easily be determined from available data.
In still another embodiment, either the amplitude or the factor is determined by means of at least one value which represents compressibility of the fuel. As a result, different environmental conditions and their influence on the compressional vibrations can be taken into account.
According to another feature of the invention, the at least one value is the speed of sound in the fuel or the modulus of elasticity or density of the fuel. These values are easily available, so that the compressibility is easy to determine.
Finally, in yet another embodiment, the value of the pressure is determined from the amplitude and phase position or the standardized value of the pressure deviation and the factor. This value of the pressure is added to the rail pressure to form a summation pressure, and the opening time of the injection valve is determined from the characteristic diagram. This summation pressure (as the common-rail pressure) and the demanded fuel quantity form the input quantities. In this manner, it has been found that compressional vibrations can be taken into account by means of previous injection operations at comparatively low expenditures.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.